The present invention relates generally to a converter systems, and more particularly to a converter system having a harmonic blocking transformer, including a main transformer coupled to a harmonic blocker, for use with conventional or enhanced converters, such as may be used in industrial or power systems applications to convert alternating current (AC) power into direct current (DC) power and vice versa, and more particularly for use in an advanced static VAR (volt-amperes-reactive) generator (ASVG). A method is also provided to reduce undesirable harmonic currents in a converter system for converting power between an AC source and a DC source.
In the past, three phase bridge rectifiers have been used to rectify three phase AC power into DC power. However, the rectified DC power has an undesirable ripple component. This ripple component is equal to six times the fundamental frequency of the AC power which was rectified. Some of this ripple may be eliminated using a three phase transformer having two sets of secondary windings each connected to a separate three phase bridge rectifier. By having one set of the secondary windings connected as a wye and the other connected in delta (referred to as wye/delta secondary windings), the resultant ripple produced by one rectifier is equal and opposite (180.degree. out of phase) to that produced by the other rectifier.
For instance, if the AC line frequency is 60 Hertz (Hz), each rectifier produces 360 Hz ripple (6.times.60 Hz=360 Hz). The 30.degree. phase shift provided by the wye/delta secondary winding connections places the ripple output of the rectifiers 180.degree. out of phase. Since the two rectifiers are connected in series, the overall ripple produced is 720 Hz, and the device is referred to as a 12-pulse converter (12.times.60 Hz=720 Hz). The 720 Hz ripple is at a lower amplitude than that produced by each of the rectifiers separately, due to the equal and opposite nature of the ripple produced by the rectifiers in conjunction with the wye/delta transformer connections. This results in a smoother DC waveform, having less undesirable ripple. However, because the two rectifiers are connected in series, the DC voltage disadvantageously is twice that which would have been produced by one rectifier.
To rectify the AC power into DC power at a lower voltage using the same type of transformer having wye/delta secondary windings requires connecting the two rectifiers in parallel. However, the parallel connection of the rectifiers requires an interphase transformer to link together the rectifier outputs so their output voltages are identical. Once again, the resulting ripple produced by the two parallel rectifiers is at 720 Hz rather than 360 Hz, due to the wye/delta secondary winding connections of the transformer. Thus, with the rectifiers connected in parallel, not only is the ripple reduced, but a desired lower DC output voltage is also obtained.
A typical transformer arrangement for a 12-pulse voltage-source converter employs two magnetic structures in each phase. The windings from each of the two magnetic structures are connected in series on the AC line side. If the AC line is rated at a high voltage, this series connection is expensive due to higher electrical losses and the high voltage bushings required for each winding. Furthermore, the use of two separate magnetic structures is very expensive, both in terms of initial cost (materials, labor, etc.) and operating cost (e.g., hysteresis losses).
A conventional wye-wye/delta transformer (that is, a transformer having a wye primary winding connection and two sets of secondary windings, one connected as a wye and the other as a delta) is often used with a 12-pulse voltage-source converter. In this arrangement, circulating currents flow in the converter-side wye/delta windings at predominantly the fifth and seventh harmonics. These harmonic currents contribute to transformer losses and degrade the quality of the DC power produced. Some methods of reducing the undesirable fifth and seventh harmonic currents involve the staggered phasing of parallel inverter bridge legs using large and expensive interphase transformers. Other methods involve reducing these undesirable harmonics using 3-level controlled bridge legs. However, these methods are both ineffective and expensive uses of such harmonic controlling devices, which could be more efficiently used to reduce other harmonics, such as the eleventh and thirteenth harmonics.
In other systems, a voltage source inverter (VSI) may be used to couple a DC system, such as a battery, an uninterruptable power supply or a battery energy storage system, with an AC line source. Such a system requires independent control of the AC/DC voltage ratio to match the DC side of the VSI to the DC source and the AC side to the line. In this system, square waves are typically applied to the two ends of each inverter-side transformer winding. The AC/DC voltage ratio is controlled by adjusting the phase difference between these square waves to produce an adjustable width quasi-square wave across the winding. Harmonics are reduced by employing multiphase systems with zig-zag transformer connections on the line side. However, such a transformer arrangement is very complex, and only acceptable for relatively small low-voltage systems.
For static VAR generator applications, the DC side of an inverter is not necessarily tied to a fixed voltage source. Therefore, the DC voltage of the inverter can be controlled to regulate the VARs. The phase adjusting or phase-shift technique discussed above may be used as a relatively inexpensive method of harmonic reduction, such as in a quasi-harmonic neutralized inverter (QHNI). For example, the phase displacement may be set at a fixed angle or stagger, such as .+-.7.5.degree., to reduce the eleventh and thirteenth harmonics. However, the QHNI transformer disadvantageously requires a delta connection on the primary high volt (HV) line side. Furthermore, if the QHNI transformer is constructed as two three-phase units, the series connection on the line side disadvantageously requires six additional expensive high-voltage bushings, as well as extra winding insulation.
Another more conventional 12-pulse inverter may be used, but suffers the same disadvantages as the QHNI device if two three-phase transformers are employed. However, this conventional 12-pulse converter may be used without reducing harmonics by any phase-shift technique. Thus, access to both ends of each inverter-side winding is not required. Furthermore, the transformer delta connection may be moved to the inverter side, so that both windings on the line side are effectively connected in wye. This construction allows the transformers to be constructed as three single-phase units, with two cores housed in each tank so that the high-voltage series connections may be made internally within the tank. Thus, one additional single-phase unit may be purchased as a spare, to provide a more cost-effective and reliable system than one requiring the use of a single three-phase unit. In this configuration, only the usual three high-voltage bushings are required, as opposed to nine HV bushings with the QHNI device. Furthermore, a neutral point for grounding use is provided on the high voltage side of each single-phase unit.
This conventional 12-pulse inverter can use a transformer having three single-phase units which are either core-type transformers or shell-type transformers. Either type of transformer may be constructed with a common path for the 60 Hz fundamental flux in the windings combined with a small magnetic shunt for the fifth and seventh harmonic fluxes and their higher order components. This construction requires less iron than that required for two separate cores. Although multi-aperture cores have been used to provide harmonic flux paths in transformers for voltage-source inverters, such as in aerospace applications, larger sizes of multi-aperture cores are expensive and difficult to construct for utility applications.